1. Field of the Invention
This invention relates to radar signal processing techniques. In particular, the present invention relates to techniques for correcting measurement errors associated with radar returns from extended targets.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
In conventional monopulse radar systems, the angular location of a target is measured by comparing the responses of a pair of ostensibly substantially identical receiver channels to radar returns from the target. However, differences in gain and bandwidth arise between receiver channels as a result of variations in parameters of the physical components actually utilized in each channel. These component variations may induce disparities in the transient responses of the receiver channels, thereby leading to errors in measurement of angular location. These errors are compounded a the size of the range gate (region of space under surveillance) approaches the size of the target. Such targets may be said to "extend" throughout the range gate.
Errors in measurement of angular location are also dependent on the accuracy of the monopulse receiver system's range-tracking capability. In particular, the output of the monopulse receiver channel pair is contemporaneously sampled when returns from a predetermined position within the range gate are expected to arrive at the receiver. To the extent that imperfect range tracking results in the target occupying a location in the range gate other than this predetermined position, the returns therefrom will arrive at the receiver (arrival time) at a time other than that expected. It follows that the time the outputs of the receiver channel pair are sampled (sampling time) relative to arrival time will not be constant if inaccuracies in range-tracking are present. Moreover, the differential transient responses of the receiver channels will cause the difference in magnitude between the respective outputs thereof to vary as sampling time varies relative to arrival time. Since target angular location is determined by utilizing the relative magnitudes of the channel outputs, the apparent angular position of the target will change in response to deviations in the time interval between sample time and arrival time. Thus, imperfections in the accuracy of range tracking in conventional monopulse receiver systems exacerbate errors in measurement of angular location.
It is noted that range track errors, and therefore angular measurement errors, may become particularly acute when multiple frequencies are used in radar surveillance of a complex target. Specifically, complex targets include a number of surfaces from which radar pulses are scattered. When radiation of multiple frequencies is used to illuminate such a target the reflections therefrom can combine in a manner such that the range centroid of the target appears to vary over the physical extent thereof. Consequently, further range track error may be introduced.
Hence, a need in the art exists for a monopulse receiver having an angular measurement capability substantially unaffected by mismatches in channel transient response or target range tracking inaccuracy.